1. Field of the Invention
This invention relates to a metal gasket interposed between opposed surfaces of a cylinder head and a cylinder block so as to seal these surfaces.
2. Description of the Prior Art
In a conventional engine, a metal gasket comprising a bead-carrying elastic metal plate is interposed between opposed surfaces of a cylinder head and a cylinder block so as to seal a narrow space therebetween. This metal gasket is formed by making combustion chamber bores in a bead plate comprising an elastic metal plate, and then forming beads along the circumferences of these combustion chamber bores. When the metal gasket is interposed between opposed surfaces of a cylinder head and a cylinder block and fixed by tightening the same by bolts, the beads provided on the bead plate form annular elastic contact portions, i.e. seal portions against the opposed surfaces of circumferential portions of the combustion chamber bores, whereby the inner and outer portions of the combustion chamber bores can be sealed.
In a modern internal combustion engine, an increase in the efficiency and an increase in an output are demanded. Various measures are taken as means for meeting the demand, which include increasing the number of cylinders to produce a multicylinder engine, increasing the combustion temperature of the gaseous mixture, or providing swirl chambers in addition to primary combustion chambers to carry out primary combustion in swirl chambers and secondary combustion in primary chambers. Therefore, the temperature in the combustion chambers increases to a high level, and the high temperature comes to influence the metal gasket as well. Consequently, it has been demanded that a metal gasket resisting a high temperature be developed.
FIG. 9 shows a metal gasket 18 applied to an engine having a swirl chamber 7. The metal gasket 18 is provided between opposed surfaces 3, 4 of a cylinder head 1 and a cylinder block 2, and coolant-contact areas 16 are formed in a position in which a water jacket 11 in the cylinder head 1 and that 12 in the cylinder block 2 are opposed to each other, the coolant-contact areas 16 being provided with holes 17 which allow the water jackets 11, 12 to communicate with each other. In a cavity 14 formed in the cylinder head 1, a hot plug 8 defining the swirl chamber 7 is positioned and set. In a diesel engine, the hot plug 8 is provided with a glow plug and a fuel injection nozzle which are not shown. Fitted in each of the bores, the number of which corresponds to that of cylinders formed in the cylinder block 2, is a cylinder liner 15, which constitutes a cylinder 13 in which a piston 5 is reciprocatingly moved. The hot plug 8 is provided with a communication port 9 via which a primary chamber 6 formed in the cylinder 13 and the swirl chamber 7 communicate. The communication port 9 and a portion therearound in the bottom surface of the hot plug 8 directly face the primary chamber 6 but the remaining crescent-shaped region is in contact with the metal gasket 18. The combustion gases, such as flames and an unburnt gaseous mixture ejected from the swirl chamber 7 and communication port 9 transmit heat to a bottom portion 10, which forms a lower portion of the communication port 9, of the hot plug 8, so that the bottom portion 10 becomes a highest-temperature region.
The metal gasket disclosed in Japanese Patent Laid-Open No. 73156/1989 will now be roughly described with reference to FIGS. 10 and 11. A metal gasket 70 can be applied to a multicylinder engine, and comprises two bead plates 71, 72 formed by elastic metal plates and having combustion chamber bores 73 and beads 74 formed so as to extend along the circumferences of the combustion chamber bores 73, heat resisting layers of coating 75 provided on outer surfaces of the bead plates 71, 72, and an intermediate plate 76 inserted between the bead plates 71, 72. The bead plates 71, 72 are provided in the portions thereof which are around the beads 74 with coolant-contact areas 78 which the cooling water passing through the water jackets in the cylinder head or cylinder block contacts. When the metal gasket 70 is tightened between the opposed surfaces of the cylinder head and cylinder block, the nonplanar characteristics of the opposed surfaces is offset by the heat resisting layers of coating 75 to improve the sealing performance of the metal gasket. The holes 78 for communicating the cooling water jackets in the cylinder head and cylinder block with each other are provided sporadically in belt-like regions 79 which annularly surround the circumferences of the combustion chamber bores 73. The belt-like regions 79 as a whole are surrounded with parallel-extending beads 74, 77, whereby the cooling water does not leak to the outside of the belt-like regions 79. In order to prevent the separation of the layers of coating of sealing material and the circulation thereof with the cooling water in this metal gasket 70, the portions of the layers of coating which are on the belt-like regions 79 including the holes 78 are removed in advance.
In general, the metal gasket 70 is made in accordance with a design of the shape and construction of water jackets which is determined taking into consideration the conditions including the capability of cooling the cylinder head and cylinder block, the capability of removing rust from the water jackets, the capability of carrying out a cooling operation uniformly and effectively and the capability of preventing a decrease in strength. In order to maintain the cooling condition of an engine as a whole evenly and optimumly, it is not preferable to cool the engine as a whole excessively. Accordingly, the quantity of cooling water flowing between the cylinder head and cylinder block is regulated by properly determining the size of the holes 78 in the metal gasket 70. The size of the holes 78 of the metal gasket 70 is generally set smaller than that of an opening of a cooling water passage in the cylinder head or cylinder block. However, in a certain type of metal gasket, the holes are not provided, and such a metal gasket is placed in a closed state.
In the metal gasket 70, the sealing of the belt-like regions 79 including the holes 78 is done by the beads 74, 77 formed in parallel with each other so as to enclose the combustion chamber bores 73. In general, the beads for the holes formed on the bead plates in accordance with the shape of the opening of the cooling water passage in the cylinder head or cylinder block are set to annular structures in the positions scattering on the bead plates, i.e., on the circumferential portions of the combustion chamber bores so as not to cause the cooling water to leak widely to the outer surfaces of the bead plates of the metal gasket, and the bead plates to be corroded.
The metal gaskets which have been proposed include a metal gasket 70 formed as shown in FIG. 11 by inserting one intermediate plate 76 between bead plates 71, 72 comprising elastic metal plates, so as to offset the irregularity of the opposed surfaces of a cylinder head and a cylinder block more easily. Recently, a metal gasket 80 having two intermediate plates 86, 87 between bead plates 81, 82, i.e., comprising a structure of four plates in total as shown in FIG. 12 has been proposed (refer to, for example, Japanese Utility Model Laid-Open No. 66457/1992).
When a conventional metal gasket is used in an engine, a sectional surface, which constitutes an inner circumferential surface of a combustion chamber bore, of an intermediate plate, if any, and also the corresponding sectional surfaces of bead plates faces a combustion chamber, are directly exposed to a high-temperature combustion gas and receive heat from the contacting combustion gas. The heat received from the combustion gas is not rapidly radiated to the cylinder head, and it is liable to be accumulated in the interior of the metal gasket since the surfaces of the bead plates are coated with a heat resisting material. A metal gasket for a multicylinder internal combustion engine receives at the portions thereof which are close to two adjacent combustion chamber bores the supply of heat from two combustion chambers, so that the temperature of these portions is liable to become high. In a swirl chamber-carrying engine, the gases, such as flames and unburnt gaseous mixture produced after the ignition and combustion of a gaseous mixture in the swirl chambers are ejected into a primary chamber through communication ports, and the portions in the vicinity of the communication ports are specially heated with the gases, such as flames and unburnt gaseous mixture, so that the temperature of these portions is liable to become specially high.
Therefore, in a conventional metal gasket, cracks occur due to thermal stress in the portions of the bead plates and intermediate plate which are heated to a high temperature. A permanent set in fatigue also occurs in such portions due to a decrease in strength ascribed to a high temperature, to cause a decrease in the sealing function and durability of the gasket. Therefore, a conventional metal gasket has a problem that this bad influence upon the structural rigidity and strength thereof has to be reduced to as great an extent as possible.